Size dispersion effects during the migration of natural submicron bentonite colloids (< 200 nm) through a ceramic column are observed for the first time by laser-induced breakdown detection (LIBD) at ppm (parts per million) mass concentration. Larger size fractions (similar to 200 nm) arrive prior to smaller size fractions (< 100 nm) at the column outlet in agreement with model predictions and earlier findings with carboxylated polystyrene spheres. By addition of trace amounts of americium(III) and plutonium(IV), colloid mediated transport of these radionuclides is studied. The peak arrival times of Pu-244 and Am-241, as measured by ICP-MS, match the bentonite colloid breakthrough and occur significantly prior to the conservative tracer (HTO) indicating the colloid-borne migration of tri- and tetravalent radionuclides. (c) 2008 Elsevier Inc. All rights reserved.